Novel pressure-sensitive transfer sheet

ABSTRACT

A NOVEL TRANSFER SHEET WHICH ESPECIALLY VALUABLE FOR USE IN TRANSFERRING IMAGES ONTO BANK CHECKS AND LIKE FORMS WHICH ARE INTENDED TO BE IDENTIFIED BY AUTOMATIC READING EQUIPMENT. THE SHEET HAS A SINGLE COATING LAYER COMPRISING PARTICLES OF A SENSIBLE PIGMENT AND A SLIGHTLYPLASTICIZED WAX. THESE PARTICLES ARE HELD IN A RELATIVELY WEAK, PRIMARILY CELLULOSIC BINDER SYSTEM. THE WAX PARTICLES ARE ADVANTAGEOUSLY BELOW ABOUT 2 MICRONS IN AVERAGE DIAMETER.

United States Patent O m 3,677,817 NOVEL PRESSURE-SENSITIVE TRANSFER SHEET Richard L. Muri, Ashland, and Thomas D. Vere, Newton, Mass., assignors to Ludlow Corporation, Needham Heights, Mass. No Drawing. Filed Feb. 24, 1970, Ser. No. 13,866 Int. Cl. H01f /02 US. Cl. 117235 10 Claims ABSTRACT OF THE DISCLOSURE A novel transfer sheet which is especially valuable for use in transferring images onto bank checks and like forms which are intended to be identified by automatic reading equipment. The sheet has a single coating layer comprising particles of a sensible pigment and a slightlyplasticized wax. These particles are held in a relatively weak, primarily cellulosic binder system. The wax particles are advantageously below about 2 microns in average diameter.

BACKGROUND OF THE INVENTION This invention relates to pressure-sensitive transfer sheets, for example, sheets which may be used as carbon paper, typewriter ribbons, or the like to place images having high dimensional accuracy on articles such as checks which, eventually, will be subjected to automatic processing based on an automated reading of the aforesaid images.

(A) Field of the invention (B) The prior art Transfer sheets of the type from which pigmented coatings are transferred onto underlying paper in the form of printing and the like have been known for many years. In recent years, however, there has arisen a large data-processing industry which utilizes automatic datareading apparatus. The efficient functioning of this in dustry, and its attendant data-reading services, has requireo significant increases in the performance capabilities of transfer sheets. For example, not only is a very rapid printing capability required, but the printed character transferred to the substrate must be very nearly dimensionally perfect having a minimum of so-called hangers around the periphery thereof, voids within the area thereof, or other imperfections which will cause the characters to be unrecognizable to automatic reading apparatus.

Moreover the transfer sheets must have a reasonable degree of sensitivity so that images can be transferred therefrom without excessive pressure. Thus transfer sheets should provide means to transfer images which do not smudge, or spread and are not subject to picking and flaking in the normal handling and usage of such automatically-processed items as checks and deposit slips. A great deal of effort has gone into providing satisfactory transfer sheets: Sheets have been suggested which have a three-layer coating system in which the outer layer is a pressure-sensitive adhesive component; the middle layer is a magnetic or so-called sensible component; and the layer nearest the substrate contains a release component which facilitates the release of the coating from the substrate itself. Among the transfer sheets using such three-layer coating systems are those described in US. Pats. 2,762,715 and 3,062,676 to D. A. Newman. Such three-layered products present quality-control problems during their manufacture because of the burden imposed by the application of three successive and interdependent coatings.

Emphasis has been shifted to two-layer coating systems such as that described in US. Pat. 3,214,285 to R. R. Wissinger et al. In this patent a transfer sheet is described which has a coating layer of silicone serving as a release agent between an overcoated ink layer and its 3,677,817 Patented July 18, 1972 substrate. The silicone-based layer also serves, after its transfer to an image-receiving sheet, as a protective coating for the ink layer trapped between the silicone coating and a sheet upon which the image has been transferred.

Still more recenty an improved coating has been disclosed in US. Pat. 3,375,125 to Shenian. Shenian discloses a transfer paper comprising what is described as a single-layer coating having a gradient within the coating over which the composition gradually changes from that part of the coating adjacent the substrate to the topmost part of the coating remote from the substrate. Although purportedly obtaining a single-layer coating, Shenian finds it necessary to utilize a two-step coating process in forming his transfer sheet. The coating on the Shenian transfer sheet comprises a diethyleneglycol ester of a hydrogenated rosin in a ketonic solvent and a highly-plasticized wax component.

There has been a steady improvement in the quality of the coatings evolved during the past few years, but there is still need for an economic coating system which provides characters of improved dimensional stability when the transfer sheet is used to identify the commercial forms which are eventually to be subjected to automatic data processing. Large banking institutions are still compelled to employ a sizable number of people whose function is to monitor checks which have been rejected by automatic data processing equipment. The rejection of these checks takes place because the characters imprinted thereon are so poorly defined that they cannot be read and are, consequently, rejected by automatic means. The employees must read the checks visually and then see to it that the checks are processed manually through the proper channels.

Moreover, it would be highly desirable if a way could be found to avoid the necessity of using a multi-coat process in forming such improved transfer sheets. Avoidance of multi-step coating processing is desirable because normal commercial means for forming rnulti-coated papers are (1) to pass it through a coating machine twice or (2) to use a single coating machine having a number of coating stations. In the former method, twice the amount of machine time is required. The latter multi-station technique is seldom used because the expense of properly instrumenting the machine so that one could monitor the precise characters of one coatin before proceeding with a particular control of the second coating step is usually prohibitive.

SUMMARY OF THE INVENTION Therefore it is the principle object of the invention to provide an improved transfer sheet suitable for use where highly faithful reproduction of characters is required.

Another object of the invention is to provide an improved coating formulation whereby such transfer sheets may be produced.

Another object of the invention is to provide a novel and improved process for producing such improved transfer sheets.

A further object of the invention is to provide an improved process for forming a transfer sheet wherein only a single coating need be applied onto the transfer sheet.

Other objects of the invention will be obvious to those skilled in the art on reading the instant application.

Applicants have achieved the above objects by forming a transfer sheet from an improved coating composition which, surprisingly, has the additional attribute of being applicable to the substrate in a single step.

The coating composition of the invention comprises (1) a liquid component which acts as a solvent for (2) a solute component and as a relatively inert dispersing vehicle for (3) a dispersed solid component.

The dispersed solid component comprises a sensible pigment and a substantially, non-plasticized, hard wax component. The solute component comprises a resinous binder system, surfactants including a resinous dispersing agent which also functions as a part of the binder component and, desirably, an oil-soluble liquid surfactant.

Without wishing to be bound by the theory, it is believed that a very important aspect of the invention is the fact that the major, hard, non-tacky, wax component of the aforesaid solid component is maintained in a substantially non-plasticized particulate state during the coating process and is only slightly plasticized during a subsequent drying step. This allows the wax component to contribute materially to the ability of a coating to fracture without formation of hangers when that coating is impacted for transfer to an image-receiving surface. Moreover, maintaining the wax in such a slightly-plasticized state has important ancillary advantages also. For example, transfer sheets having coatings prepared according to the invention have a less tendency to stick or block at high temperatures, or in storage, than do prior art transfer sheets which utilize coatings having a highly plasticized wax component. Although applicants do not wish to be bound by the hypothesis, it is thought that some small additional plasticizing of the hard wax component may occur at the moment an impacting of the sheet takes place, i.e. during the image-transferring step.

The term substantially non-plasticized as used herein to describe the hard wax means that the coating incorporating carnauba wax exhibits facile release properties from Pliofilm (selected as a standard, for convenience) which cannot be achieved when oil loadings as high as of mineral oil (based on the weight of the plasticized wax) are used to plasticize the wax according to the usual method is that disclosed by Shenian wherein the wax is fused and intimately-mixed with the mineral oil. In fact, Shenian uses a 50-50 weight ratio of Wax and mineral oil all this oil being infused into the wax in the melt state before the comminution thereof. Applicants have found that such high ratios of mineral oil (1) make difficult proper comminution of the wax particles to the 2-micron range and (2) cause such a high afiinity of any resulting one-coat system for its substrate that most desirable substrates will not form a transfer sheet that will function suitably.

The term slightly-plasticized as used herein to describe the hard wax means the state which the substantially non-plasticized wax achieves after it is coated, and dried. Normally the drying step will increase somewhat the intimacy with which the hard wax and the mineral oil are combined. Nevertheless, the total plastication of the wax remains below that which causes excessive bonding between the coating and the substrate. While the above improvement in plasticizing procedure is described with respect to a particular system, those skilled in the art will readily understand on reading this application that the same principles can be applied to use of various waxes and, indeed, one advantage of the invention is that it considerably increases the number of hard waxes that can suitably be used in formation of transfer sheets.

The improved coating of the invention provides superior performance in a number of respects. First, the uniformity of the coated product is excellent because of the quality control to which a single-coat process can be subjected. Moreover, the product of the invention has superior marking properties especially with respect to good edge defini-' tion, has non-voiding properties, and is scuff and flake resistant.

The coating contains the pigment and hard, resistant wax as solids dispersed in the liquid-, preferably isopropanol', based vehicle; the other components are substantially solvated in the isopropanol, but some of these other components may not be soluble in the isopropanol alone, but are soluble in the isopropanol-based vehicle including the other solutes.

The most advantageous solvent for use as a vehicle in the coating composition is isopropyl alcohol because of its high evaporation rate and, especially, because of its balanced solvating properties. It dissolves certain of the ingredients of the coating formulation but does not dissolve or plasticize the hard wax component. Moreover, isopropanol has the property of forming relatively weak films with cellulosic binder materials and this results in a more dependable image-transferring action when cellulosics are used as binder components. The isopropanol is used manipulatively and does not itself contribute any significant properties to the final product, except those properties derived from isopropanols solvation characteristics. Therefore all functionally equivalent solvents can be used in forming a coating composition acording to the invention.

The most advantageous major binder component is ethyl cellulose, preferably the 10 cps. standard material, e.g. as supplied by Dow Chemical Company. This material has an ethoxy range of 48.0 to 49.5. Other materials which can be used as full or partial substitutes for ethyl cellulose include cellulose acetate, cellulose butyrate, cellulose propionate, cellulose nitrate and other filmforming thermoplastic or thermosetting polymeric or resinous materials which can be combined with the other ingredients to form a releasable coating composition.

Another resin which is utilized in improving the tack retention and adhesion characteristics of the binder is a polar dispersing agent such as that phenol-modified cumarone-indene resin sold under the trade designation Nevillac Hard by Neville Chemical Company. It is characterized by a ring and ball softening range of from 70 to C. A glycerol ester of a highly hydrogenated (i.e. highly stabilized) resin, such as that sold under the trade designation Feralby Hercules Corporation, may be utilized as a full or partial substitute for the Nevillac Hard resin. This resin is particularly advantageous in improving the sensitivity of the transfer papers prepared according to the invention. It may be used as a partial or a full substitute for the phenol-modified cumarone-indene resin. Still another such polar resin is that sold by Hercules under the trade designation Foral-lOS. This is a reaction product between a highly stabilized (i.e. highly hydrogenated rosin and pentaerythritol.

The advantageous hard, relatively non-tacky wax which is included in the coating in a substantially non-plasticized form is advantageously that vegetable wax known as carnauba wax. This material mainly is composed of myricyl cerotate and has a melting range of from about 83 to 88 C. The particular advantage of carnauba wax is 1) its ability to maintain a hard, but non-tacky, character in the presence of considerable oil and (2) its insolubility in the favored isopropyl alcohol solvent. However, other waxes, especially vegetable waxes such as candelilla wax and mineral waxes such as montan wax can also be used in the process of the invention.

The advantageous fracturing and transfer properties of the coating of the invention are believed to be largely attributable to the very small particle size of the wax component. These particles should be less than five microns in average diameter, but are advantageously about 2 microns or less in mean diameter. This small particle size results in an enormous increase in wax surface area and makes possible some plasticizing of the wax under relatively mild conditions-that is during the drying and/or pressurized image-transfer steps.

The mineral oil, advantageously utilized as the latent plasticizer in the composition of the invention, may be replaced with equivalent materials the selection of which will have to be based upon the particular binder or wax with which the plasticizer is to be compounded. Vegetable oils such as castor oil and synthetic oils such as silicone oil can be incorporated into the composition of the invention under favorable conditions. Normally, however, any significant changes in formulation must "be carefully evaluated with respect to the effect they will have on the performance of the transfer sheet; some compensating change in the composition will often be required to duplicate the favorable performance characteristics of the specific formulations described herein. A white mineral oil of relatively high viscosity and high specific gravity, e.g. that sold under the trade designation Drakeol 35 USP and having a viscosity of about 360 Saybolt Seconds Universal at 100 F. and a specific gravity of 0.875 to 0.883 at 77 F., has been found to be highly satisfactory. The high gravity and viscosity tend to facilitate the comminuting action when it is carried out in a liquid medium according to the preferred method which is described below.

Any number of oil-soluble surfactants can be utilized, but a non-ionic material such as that usually sold for pigment grinding applications, under the trade designation Tenlo 70, by the Nopco Chemical Company Division of Diamond Alkali is entirely suitable.

The particulate sensible material incorporated into the coating composition will ordinarily be of the type which can be readily sensed by automatic reading means. Thus the black iron oxide material sold by C. K. Williams under the trade designation lRN-lOO is entirely suitable for use in most check-identification applications using magnetic-type sensing means. Colored dyestuffs can also be used in appropriate applications as can light-reflective pigments such as metal powders.

One quick empirical test for an adequately dispersed and sized wax is the use of the so-called Hegman test wherein a film of the coating of the invention is dnawn down into a progressively thinner film until the larger particles or agglomerates within the film become visibly noticeable. This thickness of film is given a Hegman number which is used in the paint and paper coating industries as an indication of whether or not dispersed particles have been sufficiently comminuted. The scale is such that the larger Hegman number, the smaller is the particle size being detected by the test. In the process of the invention, it is desirable to achieve a Hegman number of about 8 connoting a very small maximum particle size. [In using this empirical test, it should be remembered that it is the largest particle (or agglomerate) present in significant quantities that is being detected, not an average or mean particle size. The mean particle size would usually be well below the size of the largest particles detectable in significant quantities.

The comminuting of the wax is advantageously done while it is in a substantially non-plasticized state. This is believed to be an important factor in enabling such a particle size to be reached while grinding the mixture for 68 hours at below 100 F. in a recirculating fluid vehicle while the wax is being impacted by slurried grinding bodies.

It has also been discovered that particularly advantageous coating compositions can be prepared when small quantities of carbon black, e.g. about 2 to 5% based on the weight of an iron oxide pigment, are incorporated in the coating formulation. The carbon black should not be highly oil absorbent; it should be selected to have relatively low oil absorptivity. Gas furnace blacks such as those sold under the trademarks Sterling by Cabot Corporation and acetylene blacks are particularly advantageous but oil furnace blacks '(eg. that sold under the trade designation Regal 99 by Cabot Corporation) can also be used and some advantage can be obtained from use of small quantities of channel black.

Although it is not understood precisely how these small quantities of carbon black improve the coatings of the invention, it has been observed that their presence does aid edge definition, result in fewer voids and improve sensitivity of the coating to pressure.

It has been found advantageous, in order to reach an average particle size of 2 microns or less to use that type of dispersing and grinding apparatus which is sold under the trade designation Attritor by Union Process, Inc. of Akron, Ohio. This apparatus comprises a reservoir in which a slurry to be subjected to fine grinding is agitated in the presence of grinding bodies, e.g. steel shot of about 0.2 inch in diameter. The slurry is mixed to assure good mixture of the dispersed materials which differ so greatly in specific gravity and would otherwise tend to separate or stratify in the mixture. A cooling system is used where necessary, to keep the temperature of the slurry below that which would cause the plasticizing of the hard wax. Temperatures below about F. are advantageously maintained in this apparatus. It is usually desirable to predisperse the material in a conventional dispersing mixer before charging it to the action of grinding bodies in the Attritor.

Depending on the end-use in Which the transfer sheet of the invention is to be utilized, a large number of flexible substrates may be used to back the sheet. Among some of the sheet materials which may be used are the polyester films formed by the polymerization of ethylene terephthalate such as, for example, the film sold under the trade designation Mylar by E. I. du Pont de Nemours and Company, Inc.; the isoprene-desired polymeric materials sold under the trade designation Pliofilm by Goodyear Tire and Rubber Company, and other such films including polyethylene films, poly(vinyl chloride) films, and substrates formed of cellulosic materials. Both non-porous cellulosic films (such as cellophane) and porous cellulosic substrates (such as papers) can be used. Among the more desirable films are the 65-gauge Pliofilm P-4SF and the 40-gauge Pliofilm N-l. In general those substrates already known to be useful in the manufacture of existing transfer sheets are acceptable for use in making transfer sheets according to this invention.

Another substrate that is particularly advantageous with respect to the economics, aging characteristics and toughness of the final product is that sold under the trade designation (DB-501 by Hercules, Inc. This [film comprises an oriented and heat-set polypropylene substrate which is itself coated with a poly(vinylidene chloride), i.e. PVDC, finish coat. It is upon the finish coat that the image-transfer composition of the present invention is to be overcoated.

ILLUSTRATIVE EMBODIMENT OF THE INVENTION In order to point out more fully the nature of the present invention, the following specific examples are given as an illustrative embodiment of the present process and products produced thereby.

Working Example 1 The ingredients set forth in Table 1 below are mixed in a conventional agitated mixer to form a dispersion. Thereupon about 8 gallons of the dispersion is charged to a fluid-grinding apparatus of the type available under the trade designation Attritor 10-8 from Union Process, Inc. Steel balls of about 7 inch in diameter are used as grinding bodies. The grinding apparatus is agitated at 60 to 80 r.p.m. for 6 to 8 hours at a temperature below F. An examination of the product reveals wax particles ranging from about 0.75 micron to 3.5 microns in average diameter.

The mineral oil is that sold under the trade designation Irakeol 35 USP by Pensylvania Refining Company of Butler,

The iron oxide pigment is that sold under the trade designation IRN100 by C. K. Williams Co.

The composition so prepared, and in which all solid ingredients except the pigment and carnauba wax are dissolved, is coated on a substrate formed of 65-gauge Pliofilm P-4'SF film. The coated film is carried through a drying oven at about 60 to 80 feet per minute on a canvas belt. Belt temperature is maintained from 105 to 115 F. Air temperature in the oven is maintained below 130 F. The resultant coating weighs about 3.5 pounds per each 3,000 square feet of coated film.

The aforementioned coating was accomplished on a conventional 3-roll reverse roll coater having a metered (nip) feed feature.

Working Example 2 A number of experiments have been carried out in which transfer sheets prepared according to the invention have been compared with (A) a commonly-used transfer sheet sold by a major supplier of automatic data processing equipment, and (B) sheets prepared according to the teachings of Shenians U.S. Pat. 3,375,125. This test was carried out to determine the number of bank checks, carrying magnetic identification images transferred from the sheets being tested, which would be rejected by the automatic data reading equipment.

Number of Checks in Checks Percent sample rejected rejected Although all the percentages are small, when they are viewed in light of the vast number of checks handled in a given day by a banking institution and the fact that all rejected checks must be handled manually (at a rate of about 80 per day) by a clerical employee, it will be obvious that great advantage is obtained from using the transfer sheet of the invention.

When the test was carried out on a different automatic reading apparatus, the rejection rates gave about the same picture:

Percent rejected Check A 3.52 Check B i 1 0.12 Working Example 1 0.01

Working Example 3 The mixing and coating procedures of Working Example l were followed, but the following coating formulation was used:

The materials, where not specified, are the same as those used in the composition disclosed in Working Example 1. The magnetic iron oxide pigment was that sold under the trade designation M 4232 by the Minerals, Pigments and Metals Division of Charles Pfizer & Co., Inc., the Feral-85 has been identified earlier as a glycerol ester form of a highly-hydrogenated resin.

When coated on Pliofilm and dried, the above-identified formulation yielded a transfer sheet which possessed the essential advantages of the transfer sheet described in Working Example 1, and also was somewhat more sensitive to pressure, i.e. of increased suitability for use with forms utilizing a plurality of transfer sheets.

Working Example 4 The mixing and coating procedures of Working Example 1 were followed, but the following coating formulation was used.

Parts by wt. Magnetic iron oxide pigment 43 Ethyl cellulose a 6 Tenlo 70 surfactant -----1-------.-. l

Foral-IOS 4 Mineral oil n 16 Carnauba wax l6 Isopropyl alcohol n 288 The materials, where not specifically identified herein or elsewhere in the specification, are the same as those used in the composition of Example 3.

When coated on film and dried, the above-identified formulation yielded a transfer sheet which possessed the essential advantages of the transfer sheet described in Working Example 1 but was more like the transfer sheet of Working Example 2 with respect to its sensitivity characteristics.

Working Example 5 The mixing and coating procedures of Working Example 1 were followed, but the following coating formulations was used:

The carbon black dispersion is a material comprising about 70% isopropyl alcohol, 25% carbon black and 5% of a resinous dispersing agent. It is sold under the trade designation Alcoblak 322 by Columbian Carbon Company.

Other materials are those used in the formulation disclosed in Working Example 4 above.

When coated on film and dried, the above-identified formulation yielded a transfer sheet having the essential advantages of the transfer sheet described in Working Example 1, and also was of improved sensitivity while also allowing the fewest number of voids and imparting excellent edge-definition characteristics.

It is of course to be understood that the foregoing examples are intended to be illustrative and that numerous changes can be made in the reactants, proportions, and conditions set forth therein without departing from the spirit of the invention as defined in the appended claims.

It is to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described.

What is claimed is:

1. An article formed of a flexible substrate and a coating thereon, said article being suitable for use as a transfer sheet in the printing of characters on forms to be subjected to electronic data processing, wherein said coating comprises:

(A) a dispersed solid component including (1) a slightly-plasticized hard wax having an average particle size of less than about two microns, (2) a sensible pigment, (B) a binder component comprising (1) binder resin, (2) mineral oil, and (3) an alcohol soluble, polar resin.

2. An article as defined in claim 1 wherein said wax is carnauba wax.

3. An article as defined in claim 2 wherein said sensible pigment is a magnetic iron oxide and said binder resin comprises a major portion of ethyl cellulose.

4. An article as defined in claim 3 wherein said dispersed component additionally includes a minor quantity of carbon black.

5. An article as defined in claim 4 wherein said quantity of carbon black is from 2 to 5% based on the weight of said magnetic iron oxide.

6. An article as defined in claim 2 wherein said dispersed component additionally includes a minor quantity of carbon black.

7. An article as defined in claim 1 wherein said quantity of carbon black is from 2 to 5% based on the weight of said magnetic iron oxide.

8. An article as defined in claim 1 wherein said dispersed component additionally includes a minor quantity of carbon black.

9. An article as defined in claim 8 wherein said sensible pigment is magnetic iron oxide and said quantity of carbon black is from 2 to 5% based on the weight of the magnetic iron oxide sensible pigment.

References Cited UNITED STATES PATENTS 3,382,088 5/1968 Noda 11736.1 3,375,125 3/1968 Shenian 117-36.4 3,111,421 11/1963 Newman et al 117-235 X MURRAY KATZ, Primary Examiner B. D. PIANALTO, Assistant Examiner US. Cl. X.R. 

